Thus far, forming and erasing of images on and from a heat reversible recording medium (a medium) are carried out using a contact-type method such that a heat source is made to be in contact with the medium to heat the medium. Normally, as the heat source, a thermal head is used for image forming, while a heat roller, a ceramic heater, etc., are used for image erasing.
Such a contact-type recording method is advantageous in that, when the heat reversible recording medium is a flexible one such as a film, paper, etc., it is possible to carry out uniform image forming and erasing by uniformly pushing the medium against the heat source using a platen, etc., and it is possible to inexpensively manufacture an image forming device and an image erasing device by diverting a component for a printer for a conventional thermal paper for use therein.
However, with a contact-type recording method, there are problems of decreased density and defective erasing since, when printing and erasing are repeated, a medium surface becomes shaven, and unevenness is produced, causing a part thereof not to be in contact with the heat source such as a thermal head, a hot stamp, etc., and causing non-uniform heating.
Thus, as a method of image forming and erasing uniformly in a non-contact manner, a method of using a laser is being proposed, for example. In this method, which uses the heat reversible recording medium for a transport container used for a distribution line, writing is carried out with the laser, while erasing is carried out with hot air, warm water, an infrared heater, etc. A non-contact type recording method makes it possible to carry out recording even when unevenness is produced on the surface of the heat reversible recording medium.
As an example of such a device which carries out recording in a non-contact manner using the laser, a laser irradiating device (a laser marker or a laser marking device) is commercially available which utilizes a technique such that a laser beam is irradiated onto a medium such as metal, plastic, thermal paper, etc., to heat the medium to write thereto a letter, a number, a symbol, etc.
The laser beam may be irradiated using a gas laser, a solid-state laser, a liquid laser, a semiconductor laser, etc., as a laser beam source of the laser irradiating device to write a letter, etc., onto a medium such as metal, plastic, thermal paper, etc.
Drawing is carried out by irradiating the laser beam for heating to shave and burn the metal and the plastic. In the meantime, for the thermal paper, which has a property to change color due to heat, drawing is carried out by a recording layer developing color through heating with laser beam irradiating.
Compared with a metal or plastic medium, the thermal paper is easy to handle, so that it is widely used in a field of distribution, etc., as a medium onto which an article name or an intended address of an article is printed.
Moreover, when the heat reversible recording medium within the medium is used, the laser beam is irradiated onto the heat reversible recording medium, so that a photothermal conversion material absorbs the beam to convert the absorbed beam to heat, with which it is possible to carry out recording and erasing. As a related-art technique of image forming and erasing using the laser, a laser recording method is being used which carries out recording using a near-infrared laser beam, combining leuco dyes, a reversible developer, and various photothermal conversion materials.
Then, a technique is known which prints a two-dimensional code onto a medium using such a laser recording method.
Moreover, as shown in FIG. 1A, to draw a two-dimensional code which includes six two-dimensional code components (below, components, which are elements included in two-dimensional code components that are divided for each cell, are called two-dimensional code components), there is a method which carries out drawing by a raster scan as shown in FIG. 1C. In this method of drawing, line segments for drawing the two-dimensional code are drawn line by line. When each two-dimensional code component included in the two-dimensional code is formed by two line segments, for drawing the two-dimensional code in FIG. 1A, it is necessary to draw over four lines, so that line segments (a line segment denoted with a drawing order 1 and a line segment denoted with a drawing order 2) on a first line are drawn and then line segments (a line segment denoted with a drawing order 3 and a line segment denoted with a drawing order 4) on a second line are drawn. Then, line segments (a line segment denoted with a drawing order 5 and a line segment denoted with a drawing order 6) on a third line and line segments (a line segment denoted with a drawing order 7 and a line segment denoted with a drawing order 8) on a fourth line are to be drawn. Such a raster scan may be carried out to draw line segments of joined two-dimensional code components, with a shorter total distance for moving from a line segment drawn to the subsequent line segment, making it possible to carry out drawing in a short time (see Patent document 1, for example).
However, with related-art laser recording methods, when a two-dimensional code is drawn, there are problems that it takes a long time for printing and printing quality is poor. Moreover, these problems occur, not only with a heat reversible recording medium, but also with processing metal, plastic, etc., with a laser.
More specifically, there is a method of drawing six two-dimensional code components in drawing orders 1-12 as shown in FIG. 1B, for example. With this method, for the six two-dimensional code components included in the two-dimensional code shown in FIG. 1A, drawing of one of the two-dimensional code components is completed before moving on to draw the subsequent two-dimensional code component.
However, as the two-dimensional code components shown in FIG. 1B are drawn with two line segments, in general, it is often a case that one of the two-dimensional code components is formed of line segments on multiple lines, so that, with the drawing method shown in FIG. 1B, there is a problem that it takes time for moving to the subsequent two-dimensional code component each time, leading to a long time required for drawing in total.
Moreover, there is a problem that it is more difficult for a color to develop at a start point of each line segment that has little heat stored relative to the other parts. To draw the two-dimensional code components as shown in FIG. 2A, a gap opens with a neighboring two-dimensional code component in a line direction (shown as a horizontal direction) as shown in FIG. 2B when drawing is carried out using the drawing method in FIG. 1B unless the starting point develops color. In order for the starting point to develop color, it is necessary to irradiate a laser with a stronger drawing output. However, there is a problem that increasing the laser output for just the starting point causes a large amount of energy to be applied to the medium, leading to color development decreasing, some non-erased parts remaining, etc., and, thus, a repeated degradation in durability.
Moreover, with the method in FIG. 1C, a longer line segment for drawing joined two-dimensional components in the line direction has a larger amount of heat stored relative to a shorter line segment, causing high printing density. In other words, in order to draw the two-dimensional code components as in FIG. 3A, there is a problem, as shown in FIG. 3B, that joined two-dimensional code components end up getting printed denser relative to a separate two-dimensional code component.
Moreover, even with the method of FIG. 1C, as with the drawing method of FIG. 1B, the line segment ends up being shorter by an amount corresponding to how weak the color development is at the starting point. Then, as an impact of a phenomenon of the line segment becoming shorter is greater for a separate or shorter two-dimensional code component relative to the joined two-dimensional code components, there is a problem that, as shown in FIG. 2C, the separate or shorter two-dimensional code component ends up getting larger relative to the joined two-dimensional code components. (In other words, the separate or shorter two-dimensional code component ends up getting printed smaller relative to the joined two-dimensional code components.
Furthermore, when the length of one line of the two-dimensional code is small, or the drawing speed is fast, there may be cases where the impact of heat when drawing the previous line remains when the subsequent line is drawn. In this case, when the subsequent line is drawn, as shown in FIG. 4, a part which should not develop color in the first place that is other than the six two-dimensional code components ends up developing color, causing printing quality to become poor. In this way, a part which should not develop color in the first place ending up developing color is a problem which may occur in either one drawing method of FIG. 1B and FIG. 1C.